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You can deadhead old or dead growth. That will be fine. Those are pollinated flowers, and will turn into seed pods if left on. This article will help you with the care of these: https://www.gardeningknowhow.com/ornamental/flowers/balloon-flower/growing-balloon-flowers.htm

The verbena appears to need fertilization. This article will help you to care for them: https://www.gardeningknowhow.com/ornamental/flowers/verbena/growing-verbena-plants.htm

Yes, you can take these dead flowers off at any time. Just remove the stem close to the soil level. This article will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/flowers/balloon-flower/growing-balloon-flowers.htm

All plants produce food through photosynthesis:

https://ssec.si.edu/stemvisions-blog/what-photosynthesis

https://www.gardeningknowhow.com/ornamental/flowers/balloon-flower/growing-balloon-flowers.htm

It's a balloon plant. Here is care information:

https://www.gardeningknowhow.com/ornamental/flowers/balloon-flower/growing-balloon-flowers.htm

Unfortunately, your photo did not come through.

It sounds like the roots were disturbed while planting. This will lead to transplant shock, which can be exacerbated by overwatering in compensation for heat.

Transplants should be done in Spring to avoid heat shock.

As of now, caring for the plant and making sure to not over water will be the best course of action. There are a few things that you can try to mitigate transplant shock but these are no guarantee to the survival of the plant.

Here are some articles that will help:

https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm

https://www.gardeningknowhow.com/ornamental/flowers/balloon-flower/growing-balloon-flowers.htm

No. If they die, that will be it. Unfortunately, your photo did not come through, so I am unable to assess the damage. If there is still some life to them, you could try a fungicide with copper in it. It may or may not kill the infection. It is worth noting that they do not like to be disturbed once they are in ground, so any time that this is done you can expect them to be very unhealthy until they are able to establish again.

Regardless, you should treat the area before replanting, or planting anything else in the spot.

Here are some articles that will help:

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

https://www.gardeningknowhow.com/garden-how-to/projects/how-to-solarize-garden-beds-to-eliminate-garden-pests-in-the-soil.htm

https://www.gardeningknowhow.com/ornamental/flowers/balloon-flower/growing-balloon-flowers.htm